Electric-current transmission



Dec. 1'8, 1928.

' R. c. MATHES ELECTRIC CURRENT TRANSMISSION Filed oct. 9, 19254"sheets'sheet 1" Dec. 18, 1928` I 1,695,813

R. c. MATHEs v ELECTRIC CURRENT TRANSMISSION Filed Oc?. 9, 1923 4Sheets-Sheet 2 ffm fLMMMM/m f Vvvvwvwfm A l F/gffl MMM/wm,\/\/\/\/vvvvwm l /nvemr Habe/7* I Ala/bets Dec. 18, 1928.

l R. c. MATHEs ELEGTHIC CURRENT TRANSMISSION Filed Oct. 9, 1923 4Sheets-Sheet 5 ffl? Dec. 18, 1928. A 1,695,813

Y R. c. MA1-HESv ELECTRIC CURRENT vTRANSMISSION Filed oct. 9, 192s 4sheets-sheet 4 9,5' Amiliferd Rectfer )naw/L' fn a e if ffy@ //0 v //a I//2 TH 'Mlm 'I'I'Ir .///IT //"r To Speech Amplifier v /nvemar Rabe/f6Maf/M5 Patented Dec. I8, 1928.

UNITEDN STATES APATENT OFFICE.

ROBERT C. MATHES, OF WYOMING, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRICCOMPANY, IN CORPOBATED, OF NEW YORK, N. Y., `.A CORPORATION OF NEW YORK.

Application led October 9, 1923. Serial No. $67,437.

i its function, as for instance, by causing un-` due loss inarticulation because of the fallure of the contacts of the switchinglmeans to condition a signaling path for efficient transmission in thedesired direction in time'to transmit `properly the 'initial part .ofthe message incident upon'the path.v It has beenl suggested that such atendency may be overcome by the proper use of a so-called delay circuit,consisting, for example, of a low-pass lter type of structurecomprising' recurrent sections each having a series arm containinglumped inductance and a shunt arm contain- `ing lumped capacity. Forinstance, it has been proposed to insert such a circuit, which has theproperty of .appreciably delaying the propagation of signaling currentstherethrough, in the part of the signaling path between the point inthepath at which the switching means responsive to transmission derivesenergy from the path and the'point in the path at Which the switchingmeans conditions the path for eificient transmission. In accordance withthis invention, in order to diminish the requisite delay time of delaycircuits in suchnses, the switching means is caused to accomplish itsfunction immediately upon the openin of a contact, the opening of thecontact eing accomplished in response to signaling currents and withoutdelay due to other mechanical movement. The reason Why thisprocedure-results in vdiminution of the requisite delay time of thedelay circuit is that the time required for a relay to break, or openits contact, is far less than the time consumed in the travel of thearmature vto a make, or closing contact.

It has been discovered that Where highy speed switching means, or otherquick operatino means, responsive to transmission is used forcontrolling arepeating circuit in order that when the repeating circuitis in use'it may give good transmission in one and only one directionata time, the operation of the controlling means may be deleteriouslyeffected by transient energy which. may emerge from the line connectedto the output of the yrepeater after the input to that line has ceased.In general, when the input transmission to the line ceases lsome energywill return from it and will enter the output of the repeater. There arevarious factors which may cause energy sent out by a repeater to reenterit. For example the energy may be reflected from terminal apparatus; ormay be stored in the line and fed. backfinto the repeater, or stored interminal apparatus and fed back into the line and repeater, uponcessation of input from the repeater to the line; or may be sent backinto the line and repeater byy repeaters such for instance as ordinary,or standard, 22- type repeaters to which the line may be connected. AneXtreme case is that of a repeatered line with excessive echocurrents.In the case of a voice controlled telephone repeater, for instance, asfar as tlierepeater and its controlling apparatus are concerned thistransient returned energy acts in` the Same Way Vas do voice currentscoming from the listener..y Consequently, if the energy of to therepeater from the line into which they were last transmitted, and maycause the repeater to aga'become operative in the d1- rection in whiclitfirst transmitted and transmit some of the transient energy to the linefrom which it was originally returned. Thus,

these transient returned currents is sufficient,

of its direction of transmission. Even if the repeater be not setintosustained singing, there may be sufficient lreflection of energy tocause it to reverse its direction one o'r more times. The effects ofsuch reversals would de end upon the number of the reversals, tllfe typeof the repeater and other factors.

I n accordance withthe invention the repeater controlling means is madeto have at the end of each period of uninterrupted transmission, arelaxation time, or period of insensitivity7 to energ entering theoutput side of the repeater, t isrelaxation time being;r made suitablefor protecting the repeater from deleterious elects of transientkickbackor return of energy from the connected' lines; -for it has beendiscovered that, in general, it is possible to provide a sufiicientvrelaxation time without causingundue loss in articulation. because ofinterference with the ability of the transmission receiving station tobreak in or take control of the repeater vwith suiicient prom tness uponcessation of transmission from t 1e repeater into the line connected tothat station.

vIt has heretofore been proposed, for example in John Mills Patent No.1,434,790, November 7, 1922, to suppress echo currents in long two-wirecircuits involving bilateral repeaters, that is, repeaters in conditionatv some one time to transmit in either direction, for instance, theordinary or standard 22- type repeaters, by `emp1oying meanswhichrespond to transmission in one direction in Ithe systemin such manner asto cut oif transmission in the otherdirection. In accordance with thepresent invention, a repeater which is never in condition, at any onetime to transmit in more than one direction, an

which responds to transmission in one direction to condition therepeater for`tra'nsmitting in only that direction, may be connected in along two-wire circuit, either alone or in tandem with bilateralrepeaters, tosupf press echoes as welll as to amplify the trans` missionin either direction; and where it is desired to reduce the number ofpossible echo paths or break up echoes other than those which wouldtraverse the whole length of the circuit, a plurality of echosuppressing repeaters may be connected in the circuit, one at each point.at which it is desired to stop the passage of echo currents. The natureand effects of echo currents have been explained by A. B. Clark in hisarticle on Telephone transmission over long cables, Journal of theAmerican Institute of Electri-cal Engineers,

, January 1923, page 1, and reference may be had to that article for abetter understanding of the phenomena of echo effects.

Fig. 1 of the accompanying drawings is a circuit diagram of a repeaterembodying various features of the invention; Figs. 2, 4 and 5`are curvesfor facilitating explanation of the operation of the circuit of Figs. 1and 3; Fig. 3 shows a modification offa part of the circuit of Fig. 1;Figs. 6 and 7 show two modifications of another part of the circuit ofFig. 1; Fig. 8 is a circuit diagram of a repeater embodying variousfeatures of the invention and employing only one speech ampliie'r anddelay circuit; Fig. 9 shows a modication of a part of the circuit ofFig. 8; and

Fig. 10 shows schematically how repeaters such, for example, asthatshown in Fig'. 1 or Fig. 9, can be employed for su pressing echoesin long two-wire circuits, w ich may have bilateral repeatersr connectedtherein.

In Fig. 1 two lines 11 and 11 forming, for instance, part of a two-wiretelephone transmission'system, are interconnected by a repeater circuitcomprising an east repeater 13 for transmitting from west to east and af west repeater 13 for transmittingfrom east to west, as indicated bythe arrows. The repeater circuit also comprises the usual lthree-windingtransformers or hybrid coils 15 land 15', and comprises resistances 17and 17 in place of the impedance networks usually employed in a 22-type`repeater for balancing the impedances of the lines between which therepeater is connected. Preferably, resistances 17 and 17 approximatelybalance the impedances of lines 11 and 11' respectively; but, forreasons which will be made apparent hereinafter, close balancing is notrequired. If desired, transformers 19 and 19 ma be interposed betweenlines 11 and 11 an their hybrid coils, respectively, as shown.

' The output circuit of'repeater 13 includes the primary winding of atransformer 20.

`lThe secondary winding of transformer 20' is connected to one limb of ahybrid coil 21,

another limb of which is connected to a rel sistance 22 for balancingthe output impedance of the repeater 13. The-series winding of thehybrid coil 21 is connected to feed into aodelay circuit 25, describedhereinafter, the delay circuit in turn feeding the series Wintiing f thehybrid coil 15', that is the winding conjugate `to the bridge circuit ofthe hybrid coil. If desired, a transformer 26 may be interposed betweenthe delay circuit 25 and the series winding of hybrid coil 15', asshown.l

The output circuit of repeater `13includes theV primary Winding of atransformer 20. The secondary winding of transformer 20 is connected toone limb of a hybrid coil-\21,

anotherlimb of which is connected t0 a resistance 22 for balancing theoutput imped: ance ofthe repeater 13. Theseries Winding f the hybridcoil21 is connected to feed into a delay circuit 25 similarto the delacircuit 25, the delay circuit 25 in turn lfee ing into the serieswinding of the hybrid coil 15. If

desired, a transformer 26 may be interposed between the delay circuit 25and the series winding of the hybrid coil,15,asshown.

The output of the delay circuit is nerf -mally short circuited through alead 30, the

armature'and back contact of a relay 31, a lead 32, the armature andback contact of a two-Windin g relay 33, and a lead 34.

The output of the delay circuit 25" is norerably comprising tandemconnected electron tube amplifiers 42 and 43 feeding a high pass filter44 havinga cut-o5 frequency of the order of 500 cycles per second.l'.lhe filtermay be, for example, of the type shown in Fig. 6 of G. A.Campbell Patent No. 1,227,114, May

' 22, 1917. The filter 44 feeds a rectifier 45,

preferably of the three electrode electron tube type, through a lead 46,the normally closed contact of a relay 50', a lead 51, the normally 0.1microfarad is preferably. connected across closed contact of a relay52', a lead 53, and, ifdesired, a transformer as 'shown at 54. Acondenser 55 having. a' capacity of the order of 4 the output circuit ofthe rectifier, this output circuit including the Winding of the relay 31and also the Windingof a relay 50.

The bridge circuit of the hybrid coil 21'.

preferably comprises the primary Winding of a transformer 40", thesecondary Winding of Which feeds an amplifierand filter 41 'correvsponding to thel amplifier and filtler '41.- -The amplifier and .filter41 feeds a rectifier 45', corresponding to the rectifier 45, ,through alead 46', the normally closed contact of the relay 50, a lead 51', thenormallyclosed contact of a relay 52, a lead 53', and, ifdesiredatransformer as shown at 54". The rectifief- 454includes a condenser-(not shown) corresponding to the condenser 55, connected across itsoutput circuit, and this output circuit comprises the winding oftherelay 31' and also the Winding of the relay 50.

The right hand winding of the relay 52 and the left handwinding of' therelay 33 'are the operating windings ofth'ose relays, respectively,anda-re energized to operate those relays Whenever relay 31 closes .itsfront con-v tact, the energizing circuit extending from a groundedbattery 60 through the left hand winding, or operating winding, oftherelay 33, the operating Winding of the relay 52, the front cont-act ofthe relay 31, the head 30', and' a lead 61, to ground.

The left hand winding of the relay 52 is a holding winding, and isenergized by alocal battery 62 whenever-'the front contact of relay 33is in the,y closed condition.

`propagation of si nals.

' if desired, the right hand winding of the relay 33 `may be shortcircuited by a switch 63,

to retard the release of they armature of the relay, and a condenserwith means for varying its capacity may be connected acrossthe-operating Winding of the relay, as vshown at 64, to retardtherelease of the armature ofthe rela yzo 52', the front conta-ct of therelay 31', the lead v 30 and a lead 6l', to ground. v

Theright hand Winding of the relay 52 is a holding Winding, and isenergized `by a local battery 62 Whenever the front contact of relay 33' is in the closed condition.vv

If desired,the `left hand Winding of the relay 33' may be shortcircuited by afswitch 63',

to retard-the release of the relay armature, .and a condenser may beconnected across the operating Winding of therelay, as shown at 64', toretard the release of the relay armature.

The delay circuit 25 is shown, by way of example, asa low pass filtertype of structure, such as that of Fig. 7 of G. A. Campbell Patent No.1,227,114, May 22, 1917, Electrical receiving, translating, v-orrepeating circuit, comprising recurrent sections each having a seriesarm containing lumped inductance and a shunt arm containing lumpedcapacity. However, thedelay circuits are herepemployed as means to delaythe propagation of .signalf ing' currentstherethrough, for reasons whichW-ill be made apparent hereinafter, and they need not necessarily havefiltering properties, but may be circuits or means of any type suitableforfproducing the desired delay in the The delay .circuit 25", lil". thedelay circuit 25, is intended to represent any suitable means that maybe employed for. introducing a desired delay in the propagation ofsignals therethrough.

To reduce the effect of line noise upon the operationof the repeater,the grid potential ofthe rectifier45 should preferably be set so that adefinite minimum alternating current input tothe rectifier Will berequired before the relay 31 or the relay 50 gets sufficient recv tifiedinput to be operated; for if, instead, it be attempted to protect relay31 or relay 50 against'line noises below agiven minimum by biasing therelay, as for instance by a biasing current in an auxiliaryWinding'.(not shown) on the relay, then, noise energy being amplified insubstantially the same ratio as speech energy, substantial gain in thespeed of operation ofi-the relay is not readily obtained by increasingthe driving or operating energy, since the biasing current must becorrespondingly increased. Where the desired margin against noise issecured by choking ofi' the noise ener before it reaches the relaycircuit, the Spee of operation,v of the -relay can be made high byapplying a high value of driving or operating energy thereto.

Were the filter 44 and the condenser 55 omitted from the circuit,currents. ofvfrequenare similar to those of the amplifier and filtervcies in the neighborhood of the mechanical resonance frequencies of thetongues of relays 31 and 50 would tend to cause chattering of thesetongues when they should be held in their operatedpositions by the relaywindings, and, further, louT frequency noise currents, for instance,cross-ringing, Morse thump, and power interference,l which are likely tobe greater in magnitude than the voice currents themselves, mightdeleteriously affect the operation of the'relays. The ,filter .44prevents such disturbing currents from the rectified current to therelays.

The characteristics and functioning ofthe amplifier and filter 41.and ofthe rectifier 45 41 and of the rectifier 45, respectively.

The operation ofthe system in repeatingI from West to east will `now bedescribed? and its operation in repeating from east to west,

which is A description.

Part of the transmission from line 11 passes to the input of rep-eater13 and part is lost in the short circuit extending from similar, will beapparent from this the secondary winding of transformer 26 through leads30 and 34 and the back contacts of relays 31` and 33. A. part will alsobe lost in resistance 17. Part of the output of repeater 13 istransmitted through liybrid coil 21 and amplifier and filter 41, torectifier 45, which operates relays 31 and 50. The operation ofrelay31places a gap in the short circuit which extended across the primarywinding of transformer l26 through leads 30 and 34 and the back contactsof relays 31 and 33. Thus, the operation of relay 31V enablestransmission to pass onto vhybrid coilv 15 and thence to line 11 At theSametime relay 50 has place la gap in the output circuit lof theamplifienand filter 41', which will prevent the amplified transmissioncoming into the inputmof repeater 13 `from operating relay 31 to renderthe repeating path west operative by opening the above traced shortcircuit across the primary winding of transformer 26', or from'operating relay 50 to open the output circuit of the amplifier andfilter 41.

' By providing the delay circuit`25, the above tracedsliot circuitacross the primary winding of transformcr26`may bei opened before thetransmission from re eater 13 reaches it,

thus avoiding any loss o the beginning of'a by the delay circuit, sincethe delay circuit insuresbthat relay 50 will begin to operate anappreciable time before any possible start-v ing of the operation ofrelays 31 and 50.

Upon cessation of transmission from line 11, or when transmission fallsbelow a predetermined `energy level, relays 31 and 50 release. The valueof this predetermined level is dependent upon the energy level of linenoise or external disturbances to which the-system is subject, and uponwhat man gin or degree of protection'against such disturbancesit isdesired to afford the system. In practice,.there is a tendency for therelease o'f relay 31 to cut ofi the transmission of the last syllableofwords ending weakly, such as those with the final syllables ly and ring,Relay -33 acts to reduce this terminal clipping in'the'followingmanner.- As has been indicated' above,- when relayv 31 closes its frontcontact in response to .transmission from line 11, it causes relay 33to'placea gap. in theabove traced short circuit across the rimarywinding of transformer 26, this gap eing in series with the gap producedat the back contact of relay 31. Therefore, when the energy level of thetransmission from linel 11 falls to such a value that relay 31 releases,opening the operating? circuit of re- `lay 33 at the front contactcofthe relay 31 andclosing the gap at the back contact of the relay31, theshort circuit across the p rimary winding of transformer '26 remainsopen at the back eontact ofl relay 33 until that relay, which is slowoperatin the condenser 64 (and the ri ht i and or short circuitedwinding of the re ay, if switch 63 is closed), releases its armature andcloses' its back contact.

Another advantage which follows from the use of relay 33 is that,l iftwo of the syllables coming from line 1-1 are separated by an intervalshorter, than'fth'e time requiredfor relay 33 to close"it's'back4contact, the second syllable will find therepeater circuit alreadyoperated--temporarily biased in its favor-and due to' -hence there wouldbe no tendency toward initial clipping of this syllable.

As indicated above, when relay 31 closesits front contact in response totransmission frontline 11, it causes relay 52 to place a gap in theoutput circuit of the amplifier and filter 41', this gap being in serieswith the gap produced at the back Contactv ofrelay 50. riherefore, whentransmission from llne 11 ceases, or when the energy level of thatvtransmission fa-lls to such a value that the relays 5() and 31 release,the relay 31 opening the circuit of the operating winding of relay 52 atthe front contact of the relay 31 and the relay closing the gap at theback contact of the relay 50, the output circuit of the amplifier andfilter 41 remains open at the back contact of relay 52 until that relayreleases and closes its back contact. The release of relay 5,2. isdelayed by its left hand winding or holding Winding, the local circuitof which remains closed until the armature ofthe slow reltase relay 33has moved awayvfrom the front contact of the relay 33. The speeds ofrelease relays 33 and 52 are so adjusted that, at the contact of relay52, the output circuit of the amplifier and filter 41 remains open for ashort interval of time after the last transmission'from line 11 whichcaused relays 31and 50 to remain operated has passed through the delaycircuit 25 and onto line 11. vDuring this interval the repeatercircuit'is not responsive to energy incoming from line 11. The provisionof this time interval, or relaxation time, is to insure that energytransmitted from hybrid coil 15 into line 11 and returning from line 11to hybrid coil v15 either in amplified orunamplified form, cannotreverse the repeater, nor cause the repeater to sing between Vthe lines11 and 11 due to sustained repetition of such reversals in the mannerreferred to above.

Various factors must'be considered in determining what isa proper timeinterval to be employed as a relaxation time. If the interval is toolong, it will unduly interfere with the ability of the listener to'breakin on l the speaker, or it may even happen that the first part of aquick reply will be cut off. If

the interval is too short, the energy level of ^the transient energyreturning to the repeater from the line will, at the end of the period,be'

sufficiently high to cause reversal of the repeater. The energy level ofthe'returning transient energy at any given instant will depend upon themagnitude of the last repeated energy and the condition of the line intowhich it Was repeated. For any given energy level of the returnedtransient at the end of the relaxation period, the likelihood ofreversal of the repeater by the transient will depend upon how long,after the period of insensitivity, the means controlling the repeaterswitching means requires to attain a condition in which the subsequentoperation of the Switching means is assured. With a circuit of the typeshown in Fig. l, a swltching time of less than .003 second has readilybeen attained. By the switching time is meant the total time requiredfor ordinary voice transmission to switch the repeater from the normal01' neutral condition to the condition for repeating in one direction orthe other. The likelihood of the occurrence of repeated reversals orsinging will further ,depend upon this switching time, or the total timetaken, after the period of insensitivity, to switch the repeater vandalso upon the condition of the line into which the repeater feeds whenfirst reversed by transient energy, and the magnitude of the energy sofed into that line. Itis entirely possible that even though singlereversals from reflected energy would not take place with thetransmission of ordinary speech, a large impulse such as would resultfrom the operation of the subscribers switchhook might set the system 0Einto repeated reversals or singing.

Extensive experiments have shown that in a circuit not including anybilateral repeaters, that is, repeaters' conditioned to repeat inopposite directions simultaneously, a relaxation period of the order of.02 or .03 second'is, under ordinary conditions, suiiicient to preventreturned transient energy from causin reversal or singing of therepeater. Shoul any bilateral repeaters be included in the eircuit thisrelaxation time would have to be increased to a value of the order/of'double this value, the exact requirement as to minimum permissiblerelaxatlon time depending upon the individual case.

The shortest pause of a speaker which will permit the listener-to gainsuiicient control of the repeater to insure that the repeater will bereversed, is longer than the relaxation time of the repeater, or inAother words, is ylonger than the period of insensitivity of the repeatercontrolling means to energy entering the output side of the repeaterupon the cessation of the energy output of the repeater tothe line.Consequently,`serious i interference with the carrying on ofconversation can be avoided only if the minimum permissible relaxationtime ofv thel repeater is less than the longest silent interval that, inordinary speech, occurs as often as it is required that the listener beable to break in on the speaker. It has been discovered by extensiveexperiment, that speech, silent intervals (or intervals when the energylevel of the speech is so low that peater do occur as frequently as itis ref quired, for satisfactory conversation, that the listener be ableto breakin upon the. talker. It has been determined that satisfactoryease in ordinary ,120

the effect of the speech in preventing the lisof break can be secured ifthe relaxation time of the repeater is of the order of .05 second. ySince a relaxation time of the order of .02 or .03 second is sufficientto prevent transient return energy' from causing reversal or singing-ofthe repeater in a circuit involving no bilateral repeaters, and sincesatisfactory ease of break can be secured with a relaxation time asgreat as .05 second, a relaxation time of ab0utv.03 second may beemployed as prac4 ticable for such a circuit.

The back contact of relay 33 should close slightly before that of relay52, so that the repeater path east will be rendered inoperative vbeforethe repeater path west can be rendered operative and there willtherefore be 'no ossibility f singing of the repeater due to oth ofthese paths being operative at the saine time. Since these two paths arenever operative at the same time, the resistances 17 and 17 need notaccurately balance the impedancesof lines 11 and 11',- respectively,'inorder to prevent the repeater from singing. Neither is it necessary thatsuch 4balancing be accurate in order to insure that energy from line 11will divide between the other three impedances connected to hybrid coil15 in such manner that a satisfactory proportion of the energy willenter the repeater 13. It may be here noted that if resistance 17accurately balances line 11 the short circuit across the lower windingof transformer 26 by means of relays 31 and 33 will not affect the inputfrom line 11 to the repeater 13. If the balance is not accurate thisshort circuit will produce some effect on the input to the repeater, butstill none of importance.

It has been found that reduction of interference between the twobranches of the cir! cuit of Fig. 1 is attained by locating the relaybreaks in the amplifier rectifier systems on the grounded sides of thosesystems, as shown at the contacts of relays 50, 52 and 52 inv Fig. 1,instead of on the ungrounded sides of those systems.

In Fig. 2 the full line curve is an effective resistance-frequencycurve, and the dotted curve is an impedance-frequency curve, typical ofa delay filter such as 25 with its output short cireuited as is normallythe 'case in the circuit of Fig. 1. The vertical arrows indicate,approximately, natural frequencies of the short circuited delay circuit,and these frequencies give steady state impedance minima andthereforewould give maxima of steady state input current for constantamplitude of voltage applied to the input of the delay circuit; but-,asexplained hereinafter in connection with Figs. 4 and 5, when a voltageof anyone of these. frequencies is suddenly applied the current whichf'lows into the delay circuit rises .to its final or steady state valueslowly, in the manner indicated in Fig. 4. Therefore, if the input ofthe delay circuit and the primary1 *winding of the transformer 40 wereserially connected to each other directly across the secondary windingof transformer 20, and the hybrid coil 21 were omitted from circuit,then, for.

any oneof these frequencies, the input to transformer 40, and hence therectified current applied' to the relays 31 and 50, would,

rise-to its final value slowly, and consequently the relays would beslow operatin for any of these frequencies, since the sensitivity of theaiv relays must be kept low enough to insure' that they will not beoperated by the steady A state value of relay currents-due to noisecurrents of these frequencies. For any'frequency midway betweenfrequencies indicated by adjacent vertical arrows in Fig.v 2, the final.value of the current flowing into the short circuited delay circuitwould be small., but the current would rise rapidlyto a value evenexceeding the final value, in the manner indicated in Fig. 5. y If theinput of the short circuited circuit and the primary winding of thetransformer '40 were connected i n pan allel directly across thesecondary winding of transformer 20, the hybrid coil 21 beingv 2 the.current in the transformer 40 would p rise slowly to a lhighsteadystate value.

Therefore, in the case of either the series f -orthe parallelconnectionof the vtransformer 40 and the short circuiteddelay circuit,-tlie speed of operation of both relays 31 and 50 would be diderentforfdifferen't frequencies of voltage applied to hybrid coil 15 by linef 11, and therefore would be different for different syllables cominginto the repeater from line 11.

When the primary Winding of transformer.

40 and the input of the delay circuit 25 are connected to the secondarywinding of transformer 20 through the hybrid. coil 21 instead ofdirectly, so that the impedance of the delay circuit is in conjugaterelation to the primary winding of transformer 40 or circuit whichsupplies energy to the relays 31 and 50, the impedance of the delaycircuit will not'affect the amount of the current which flows to therelays at any frequency. 1t has no effect at any frequency,y it can haveno transient effect. Therefore, by `thus .employing the hybrid coil 21,the speed of operation of the relays can be equalized for differentfrequencies, and yet the energy ,for cont-rolling the relays can bepicked' off from the main transmission circuit` at the output side ofthe speech amplifier 13, so that is generally stepped up to as high avoltage as practicable, with the danger of driving the grid of the firsttube decidedly'positive. In such cases a degredation of speed beingtransmitted may occur, due to the introduction of harmonics. Byemploying a balanced coil', asl that at 21 in Fig. 1, such harmfulreaction can be prevented. As the amplifierrectifier is in conjugaterelation to the outgoing circuit carrying the main transmission, anyharmonics generated in the amplifier rectifier circuit are balanced outfrom that out? going circuit.

In Fig. 1 the repeater 13, the hybrid co1] 21 and the delay-circuit 25,are shown as con1-.

prised in a block 66, and in Fig. 3 is shown a block 66n which may besubstituted for the block 66 in Fig. 1. In the block 66a no hybrid coilcorresponding to the hydrid coil 21 of Fig. l is employed. The delaycircuit 25 is connectedto the repeater 13 through the transformer 20,and also through,I a transformer 67. A transformer 40, the secondarywinding of which feeds the amplifierrectifier 41-45 (not shown in Fig.3) just as the secondary winding of transformer 40 of Fig. 1 feeds thatamplifier-rectifier, has a primary winding 68 connected acrossthecircuitbetween transformers 20 and 67 and has a primary Winding 69connected across a resistance 70 which is serially connected in thatcircuit. The steady state impedance of the short circuited delay circuitpasses through wide variations (see Fig. 2) as the frequency' changes,and correspondingly its transient impedance, and at each of the lowimpedance points in the range of applied frequencies that is, at eachfrequency such as these indicated by the vertical arrows in Figs. 2) thecurrent entering the dela'y circuit, and consequent-ly the voltageacross winding 69, Varies in the general 'fashion indicated in Fig. 4,whereas when a voltage of any frequency midway between those indicatedby adjacent vertical arrows in Fig. 2 is suddenly applied the currententering thedelay circuit,I and therefore the voltage across Winding 69,varies in the general fashion indicated in Fig. 5. However, when thecurrent entering the delay circuit varies as in Fig. 4,' the voltageacross the delay circuit, and therefore aero-ss winding 68, will vary inthe generalfashion indicated in Fig. 5, wherein the time scale .is thesame as in Fig. 4; and when the current .y

entering the delay circuit varies as in Fig. 5. the voltage across thedelay circuit, and therefore across winding 68, will vary in the generalfashion in'dicated in Fig. 4. Consequently, by combining the fluxesproduced by windings .68 and 69 in transformer 40, the voltage waveapplied to the amplifier-rectifier L1:1--415 by the secondary winding oftransformer 40 can be made to be vof approximately the same form as thatapplied to hybrid coil 15 byline 11.'

In Fig. 1 there is shown a block 73 comprising the .transformer26,'hybrid coil 15',

resistance 17 and transformer 19. In Fig.

v6 there is shown .ablock 7 3al whichanay be substituted for the block73 in Fig. 1. Leads 711', 75', 76, 61, 34 and 30 in Fig. 6 are the sameas correspondingly designated leads in Fig. 1. Leads 76 and 61, whichare'the output leads from the. delay circuit, may be short circuited bythe circuit through leads 30 and 34 and the back contacts of relays 31and 33 (not shown in Fig. 6), if a short section of artificial line,indicated as 80 in Fig. 6, be inserted between the short circuited andthe junction of the input of repeater 13 with line 11'; for theartificial line 8() will prevent the short circuit from also shortcircuiting the input of repeater 13 with respect to line 11. c

The portion of the circuit of Fig. 1 which is included in the block 73may also be replaced by a circuit of the form shown in the block 7 3b inFig. 7. In the latter figure, the

transformer 26 feeds into line 11 through a balanced bridge 85, insteadof through a hylbrid coil as in Fig. 1. One side of the bridge comprisesa resistance 88 and a secondary winding 89 of a transformer which hasits primary winding fed from the transformer 26; and the other side ofthe bridge comprises a resistance 90, preferably equal to resist-A,V

ance 88, the circuit which extendsr through lead 30, thence through theback contacts of relays 33 and 31 (not shown in Fig. 7), and thencethrough lead-34, and a winding 91,

preferably having the same number of turnsv as winding 89. Normallv thebridge is in the balanced condition and'currents fed into it fromtransformer 26 produce no voltage across line 11'; b ut whenever thelast mentioned side of the bridge is open at the back.

13 and the lines 11 and 11, and also-elements 15, 15', 17, 17', 31, 31',5o and 50 in Fig. 8

are the same as in Fig. 1. Relay 152 in Fig. 8 has its operating windingin series with the front contact of relay 31 and its contact in serieswith the contact of relay 50, just as has the relay 52 in Figfl, and'isa slow release relay although the circuit of Fig. '8

has not been shown as rovided withl any relay corresponding to t e relay33 of Fig.'

1. Similarly, relay 152 in Fig. 8 has its operating winding in seriesWith the front contact of relay 31 and its contact in series ,withthe-contact of relay 50', justas has the'relay 52 in'Fig. 1, and is aslov'v7 release relay although the circuit of Fig. 8 has not been shownas provided with any relay corresponding to the relay.33 of Fig. 1.vAmpliier-rectitiers 95 and 95 in Fig. 8 correspond to theainpliier-rectiiers 41-45 and 414.5, respectively, of Fig. 1, but may,if

desired introduce greater ampliication, to a make up for the fact thatthe input to am- J pliier-rectiiers 95 and 95 is obtained directly fromthe hybrid coils and 15', re-

i spectively, Without amplification.

A balanced bridge 9G, which can be balanced .quite closely, enablestransmission from either line 11 or 11 to be supplied to the input ofrepeater 13 without get-tlng over to the other line.

The operation of the circuit in transmitting from West to east will nowbe described,

and this will make apparent the operation-in transmitting from east to.West, which is similar.

The output of the repeater 13 is normally short circuited at the contactof-relay 31 and also at the contact of relay 31"; but, as

pointed out in connection with Fig. 1, the short clrcuiting of theserles Wlndlngs of the hybrid coils 15 and 15 does not mate-` riallyinterfere ivith the supply of energy from lines 11v and 11 to the bridgecircuits of the hybrid coils 15 and 15 respectively.

ySpeech coming from line 11 Will start through the'bridge 96, repeater13and delay to open the back contacts of relays 31 and 50,

there will be no danger ofimproperly operating relays 31 and 50 orsetting up a local singing circuit. The closing of the front cont-act of'relay 31 causes the operation tof relay 152 through a local energizingcircuit.

When transmission from line 11 ceases, re'd lays 31, 50 and 152releases, but the latter relay maintains its contact open for a prede.-termined time to protect the system against .transient kick-back ofenergy from line 11.

The time 'of closing of this relay 152 is made sufficiently great togive the system a. relaX ation time of a value such as is specifiedabove for the system of Fig. 1. The relay 31 be a close balance'vfor theimpedance of line 11.

If desired rela-ys corresponding` to the relays 33 and 33 of Fig. 1 may,of course, be

incorporated in Fig. 8, to delay closure of the short circuit-s`controlled by relays 31 and 31 respectively and thus reduce terminalclipping of Words, and to delay release of rclays 50 and 50respectively. y

Instead of employing the hybrid coils 15 and 15 in Fig. 8 to prevent theshort circuits across the output of the repeater 13 from materiallyinterfering with the supply of energy from lines 11 and 11 to the inputof the repeater, artificial lines (not shown in Fig. 8) may be employed.after the fashion in which, as pointed out above, the artiiclal line 80of Fig. 6 may be employed instead of the hybrid coil 15 in Fig. 1.

Fig. 9 shows a circuit which may be substituted for the balanced bridge96 in Fig. 8, to connect theinput of the repeater 13 to the bridgepoints of the hybrid coils 15 and 15. This circuit comprises anamplifier 110, preferably of the electron discharge type, fed from thebridge points of the hybridy coil 15 (not shown in Fig. 9) through theback contacts of relays 50 and 152 (not shown in Fig. 9) and leads 111and 112, and in turn feeding into the speech amplifier 13 (not shown inFig. 9) and the circuit also comprises an amplifier fed from the bridgepoints of the hybrid coil 15 (not shown in Fig. 9) through the backcontacts of relays 50 and 152 (not shown in Fig. 9) and leads 111 and112 and in turn feeding into the speech amplifier 13 (not shown in Fig.9). Since the amplifiers 110and 110 are unidirectionally transmittingdevices, they allow energy to be supplied from either line, 11 or 11',to the input of repeater 13, but prevent that energy from gettlng overto the other line Without passing through the repeater 13,

' the delay circuit 25 and the series Winding of the hybrid coil 15 or15 connected to that other line.

Fig. 1() shows a long two-wire circuit, for' instance a telephonetransmission line, repeatered with one or more bilateral repeaters 210,for exam le ordinary or standard 22-type repeaters suc as that showninFi s. 10- or 11 of G. A. Campbell Patent No. 1,22 ,114,'May 22, 1917.One or more unilateral, voice oper- ,ated repeaters 215, which maybe,vfor example, a repeater of the type shown in Fig. 1 or of the typeshown in Fig. 8. are -also con nected `in the line 210. The dottedlinesat each side of the repeaters are to indicate that the repeaters may beelectrically or geographically remote from each other and from asubscribers station 215 atthe West end ofthe time adjusted, as pointedout in connection with the circuit of Fig. 1, to prevent energy ireturned or fed back into the repeater from the line from reversing thepoint of the repeater. Therefore, the repeater will suppress echocurrents, mitting in one direction, must have its pointing reversedbefore it is capable of transmitting in the other direction. vAsexplained in connection with the circuit of Fig. 1, the relaxation timeof the repeater should be made suiiciently great to take care of thelongest period of appreciable'returned energy or the longest echo ofobjectionably high volume or energy level, but vshould not besufficiently long to cause undue interference with the ease with which alistener can break in upon a speaker.

'Operating airelay from energyof a current having high and low frequencycomponents by separating the high frequency components from the lowfrequency components, rectifying the high frequencycom'ponents alone,removing current fluctuations of said high frequency from the rectifiedcurrent, and

. applying the resultant current to the relay, is

claimed in R. V. L. Hartley Patent 1,588,186, .Tune 8, 1926, assigned tothe assignee of this application. I

Rendering a voice controlled repeater operative to transmitin thedesired direction immediately -upon' the openings of switch contact, theopening of the contact being accomplished in response to voice currentsand without delay due to other mechanical movement, is claimed in W. S.Gorton application Serial No. 658,514, filed August 21, 1923, as-

signed to assignee of this application.

The application of a delay circuit to a. voice controlled repeater todelay transmission until the voice operated switching means has had time-to operate is claimed in H. D. Arnold Patent 1,565,302, December 15,1925, Two-way repeater circuits, assigned to the assignee of thisapplication.

The general principles herein disclosed may be embodied in manyorganizations widely different from those specifically illustrated anddescribed without departing from the spirit of the invention defined inthe apended claims. A

Although the invention has been set forth with especialreference to itsapplication to the repeating of voice frequency currents, it is ofcourse also applicable to the-transmission and repeating of currents ofother frequencies, whether higher or lower, as for instance currents ofthe frequencies commonly used for radio `and carrier currents.

The expression unilateral means, as used in the claims, refers to meansnever in condition, at any one time to transmit in more since therepeater, after trans-.A

renderingy said i transmission 1. The methodI of' operating acurrentti'ansmission device connectedl in a circuit and subJect to beingrendered operative to transmit in a given direction 1n said circuit bytransmission returning from said circuit to said device, which comprisespreventing said returning transmission from so affecting said device.

2. In a signal transmission system, a .signal transmission pathcomprising a normally closed switch contact in that condition directlyreducing the transmission eficiencyof the path, and means fortranslating received signaling variationsl into ment, and for directlymechanically employing the initial movement so produced, at itsinception, to open -said switchcontact. v

3. In combination, a circuit comprising a current transmission path,means including a closed switch and decreasing the transmittingeiicienoy of said path while said switch is closed, means variationsreceived over said path into mechanical movement, and for mechanicallyemploying the initial movement so produced, at its inception, to opensaid switch, and thereby causing increase in the transmitting eiiciencyof said path, and means for delaying transmission in said path untilsaid switchl has opened. v

4. A circuit comprising two transmission paths, means, comprising aclosed switch, shunting one of said paths and decreasing thetransmitting eiiiciency of said path, means for opening said switch inresponse to transmission in said circuit, and means for responsive meansinsensitive for a predetermined time after said other path has ceasedtransmittin 5g Thel method of operating a current transmission path anda circuit controlling device having parts so movable with respect toeach other as to increase the impedance of said device, which comprisescausing alternating signaling currents to flow in said path, rectifyingsaid alternating currents, causing said parts to so move iiiresponse toHow of said rectified currents and withoutdelay to other mechanicalmotion, and increasing the transmission eiiiciency of said pathimmemechanical movefor translating the current sol diately upon theconsequent increase of imow'of transmission in either direction in saidcircuit for rendering said device operative to transmit, but insensitiveto flow of -transmission in one direction in said circuit for apredetermined time after said device rection substantially just suicientto pre- -vent return of transmission to said device from rendering saiddevice operative to transmit.

7 A circuit, a two-way unilateral repeater connected therein, means atsaid repeater responsive to transmission of a given energy level in onedirection in said circuit for rendering saidrepeater operative totransmit in said direction in less time than the time required forenergy fed from said repeater to said circuit in the opposite directionto return to said repeater and there 'decrease to an energy level of theorder of said first mentioned level, and means for preventing saidreturned energy from deleteriously affecting the operation of saidtransmisison responsive means. f

8. In -a transmission system, a transmission path, means formingnormally substantially a short-circuit path across said transmissionpath, andmeans responsive to transmission at a partof said system otherthan said shortcircuit pathfor causing the opening'of said short-circuitpath While permitting transmission over said transmission path.

9. In combination, two unidirectionally transmitting amplifiers eachhaving an-input circuit and an output Qircuit,a closed trans-y missioncircuit connecting the outputvclrcuit of one of said amplifiers to theinput circuit of said other amplifier, a closed transmission circuitconnecting the output circuit of said other amplifier to the inputcircuit of said one amplifier, and means preventingr local singing ofsaid amplifiers, said means comprising a normally closed switch contactin one of said transmission circuits and depending upon the closedcondition of said cqnt'act for preventing said singing. f

10. In combination, a circuit comprising al current transmission path, adevice normally decreasing the transmitting 4efficiency of said path,said device having parts so ymovable as to increase its 1mpedance,'meansso movlng said parts 1n response to transmission in vsaid circuit-andwithout delay due to other 'mechanical movement, and'means fordelaying'iiow lof transmission to said path until said parts havethus'been moved.-

11. In combination, a circuit comprising a current path, a relay havingan armature, said armature in the normal unenergzed condi tion of'saidrelay closing a short-circuit decreasing the transmitting efficiencyof'said path, means for translating signaling variations into currentvariations for actuating said. relay to controlsaid armature to open-said short circuit, the opening of said short circuit increasing thetransmitting eiiiciency of said patl1,and means for delaying passage ofsaid transmission to said path until said switch/has opened.

12. A circuit comprising two oppositely directed unidirectionaltransmission paths, a switch having conta-cts'in one transmission pathwhich when closed decreases the trans# mission efficiency of said onepath, means for translating signaling variations received over 470 saidone path into mechanical movement, and l for mechanically employing theinitial movement so produced, at its inception, to open said switchcontacts, and thereby increasing the transmitting efiiciency of saidpath upon the opening of said 'switch contacts, and means for renderingsaid transmission rcsponsive means insensitive for a predetermined timeafter the other of said paths has ceased transmitting.

13. In combination, a current transmission path, a circuit controllingdevice having its parts so movable relatively to each other as toincrease the impedance of said device, means for rectifying speechvariations received over said path, means for converting theenergy ofthe rectified speech variations into mechanical movement, and formechanically employing the initial movement so produced, at itsinception, for operating said 4movable parts to increase the impedanceof said device, vand meansresponsive tov said increase of impedance forimmediately` increasing the transmission efficiency of said path.

14. A transmission network comprising a repeating path, a switch in itsclosed condiception, to open said switch thereby increasingthetransmitting efficiency of said repeating path.

15. In combination, a circuit comprising a current transmission path,means operative in response to transmission in saidcircuit forincreasing the transmission efficiency yof said. path, and means formaintaining said increased efliciency after said first mentioned meansassumes its unoperated condition.

16. In combination a circuit comprising a current transmission path,means operativev in response to transmission in said circuit forrendering said path operative to transmit, and a relay controlled bysaid means for maintaining said increased efliciency after said firstmentioned relay 'as returned to its unoperated condition.

.17. In combination, a clrcuit comprising a current transmission path, aclosed .switch decreasing the transmission efiiciency of said y-tlonreducing the transmitting efficiency of and means for maintaining saidpath at said v of the supply of signaling variations to said ath.

18. The method of reducing echo effects in a transmission circuitrepeatered with bi? lateral repeaters, which comprises maintaining saidsystem normally inoperative to repeat from either end of the circuittothe other, and upon flow of transmission in either direction in saidcircuit conditioning said circuit for transinit-ting in that direction,and holding it conditioned in that direction for a predetermined timeafter said transmission has ceased.

19. A telephone transmission system comprising a two-way unilateralrepeater and a bilateral repeater in tandem.

v20. A telephone transmission system coinprising a normally inoperative,transmission controlled, two-Way unilateral repeater and a bilateralrepeater in tandem.

21. A current transmission system coinprising bilateral repeaters and atransmission controlled, echo-suppressing unilateral repeater in tandem,said repeaters being geographically remote from each other, and saidunilateral repeater having a relaxation time atleast as great as thetime required for the longest echo which said unilateral repeater is tosuppress to pass from the repeater to -tlie reflecting point and back tothe repeater. v

22. A circuit, an impedance for approximately balancing the impedance ofsaid circuit, tWo unidirectional currentvpatlis, a hybrid coilconnecting said paths to said circuit and said balancing impedance andin conjugate relation to each other, one of said paths transmittingtoward said coil and the other path transmitting from said coil, -aswitch short circuiting said coilwith respect to said one path, andmeans for openingsaid switch in response to flow of transmission in saidone path. "f

23. A circuit, two transmission paths con-l nected to one end of saidcircuit, means substantially short circuiting one of said paths forpreventing transfer of energy between said paths, means for transmitting-to said circuitenergy flowing towards said paths, and means foravoiding great loss of energy iii transmitting from said circuit to saidotherl path.

24; In combination, a. line, two unidirectionally transmittingamplifiers therein, each having an input circuit and an output circuit,a short circuit across the output circuit of each amplifiery forpreventing local singing of the amplifiers, and a hybrid coil at theoutput side of each amplifier for avoiding great loss of incoming energybetween the line and the input circuit of the other amplifier.

25. In combination, a circuit comprising a,

current path including a normally balanced bridge decreasing thetransmitting efficiency of said path, said bridge including a'switclifor controlling the balancing thereof, and

means responsiveto transmission in said circuit for controlling saidsWitcli.

26. In combination, a line, two unidirectionally transmitting amplifierstherein,-

each having an input circuit and an output circuit, a short circuitacross the output circuit of each amplifier for preventing local singingof the amplifiers, and an impedance line at the output side of eachamplifier for avoiding great loss of' incoming energy between tlie lineand the input circuituof the other amplifier. Y

.27. In combination, in a transmission circuit and incoming line, anoutgoing line, an energy abstracting device tending to introduceharmonics into the circuit, a net work simulating' the impedance of theincoming line, and a bi-conjugate network associating all of saidelements together so that the incoming line and its network are'conjugate, and the harmonic producing device and the outgoing line areconjugate.

28. In combination, a unidirectionally transmitting circuit, animpedance balancing net-Work therefor, a reactance, a voice currentresponsive switch, an operating circuit for said switch, and means sointerconnecting said first circuit, 'said balancing network, saidreactance and said operating circuit that said reactance and saidoperating circuit areA conjugate to each other. v 29. In combination, anamplifier, an impedance for balancing the impedance presented by theoutput side of said amplifi er, a

reactance, a voice operated switch, an operating circuit forsaid switch,and means so in-v terconnecting said output side of said amplifier, saidbalancing impedance,said reactance and said operating circuit that saidreactance and said operating circuit are conjugate to eachother.

30. In combination, a switch, an operating circuit-therefor, aunidirectionally transmitting circuit comprising a filter and means forconnectingthe input end of said operating circuit in conjugate relationto the input end of said filter. f

31.' In combination, a unidirectional circuit comprising an incomingline andan o utgoing line, and aI rectifiercircuit comprising branchesconnected in serial relation and 1n shunt relation, respectively, vtosaid first circuit.

cui for rendering said path'cperative, at

different times, to transmit in both directions, in said circuit,wherebytwO-way transmission may be had, vand means for preventingtransmission pturning from said circuit l'to said path from operatingsaid transmission responsive means.

33. A transmission system comprising in combination, a circuit, aplurality of transmission paths connected thereto, each including aclosed switch contact in that condition directly reducing thetransmission efficiency ofthat path, and means for translating sigmalingvariations 'received over one of said paths into mechanica-l movement,and for directly mechanically employing the initial movement so roduced,at its inception, to open a selected) one of said contacts thusincreasing the transmitting efficiency of the corresponding path.

34. In combination, a unidirectionally transmittingpath, two circuitsfor feeding signaling Waves into said path, and means connecting sai-dcircuits to the input end of said path and in conjugate relation to eachother.

3. In.- combination, a unidirectionally transmitting path, two circuitsfor feeding signaling Waves into said path, .and a balanced bridgecircuit connecting said circuits to the input end of said path and inconjugate relation to eachother.

36. A system comprising a source of current having a relatively lowfrequency componentand a relatively high frequency component, arectifier, a filter for suppressing said low frequency componentconnected bctween sal-d source and said rectifier, a trans- Alatingdevice fed by said rectifier, and noninductive means connected totheoutput circuit of said rectifier for smoothing out said 'rcctlfiedcurrent.

37. A system comprising a source ofcuri rent having a relatively lowfrequency component and a relatively high frequency com-`` tween saidsource and said rectifier, a normally closed direct current relayoperative to open its contact in response to current from saidrectifier, and means connected to the output circuit of said rectifierfor smoothing out said rectified current, said means consisting solelyof a condenser.

38. In combination, a transmission circuit, subject to noise currents, adirect current electromagnetic relay, an electron discharge rectifierfed from said circuit and feeding said relay, said rectifier comprisinga control electrode and means maintaining said electrode at such apotential that said noise currents cannot be propagated through saidrectifier.

In Witness whereof, I hereunto subscribe my name this 2nd da'y ofOctober A. D., 1923.

ROBERT o. MATHES.

